In order to provide a vehicle offering good handling characteristics, it is important that the braking system belonging to the vehicle works satisfactorily. Included in this is that, when the brakes are applied, braking devices integral to the vehicle must distribute the braking force in proportion to the axle load. Uneven distribution of braking force can lead, firstly, to increased wear and, secondly, to the vehicle skidding or becoming unstable should the braking force distribution be very uneven. Where a trailer is coupled to the vehicle with a draw bar, furthermore, the braking force should be distributed between tow car and trailer so that the braking devices of the tow car brakes the mass of the tow car and the braking devices of the trailer brakes the mass of the trailer. For a tow car with semitrailer, some of the weight of the semitrailer will end up on the axles of the tractor. In this case, the tractor brakes some of the mass of the semitrailer.
Despite the general desire to distribute the braking force evenly between the braking devices of the vehicle, it may in certain cases be desirable to distribute the braking force differently between different axles of the vehicle. An example of such occasions is when the service brakes need reconditioning and the reconditioning is carried out by virtue of the fact that braking in which the absorbed quantity of energy at the service brake shall exceed a certain limit value. Such a system is described in Swedish patent application SE 0101253-3, the disclosure of which is hereby expressly incorporated by reference in its entirety.
Both in systems in which the braking force is to be distributed evenly between the braking devices of the vehicle and in cases in which uneven distribution is realized with a view to reconditioning the service brakes, it is important for an estimate to be made of applied braking force for braking devices integral to the vehicle.
A system for monitoring the distribution of braking force between different brakes in a vehicle is described in U.S. Published patent application Ser. No. 2002/0138189, the disclosure of which is hereby expressly incorporated by reference in its entirety. The system described in this patent application comprises a temperature sensor which measures the temperature of the surroundings of the brake. In the braking operation, the brakes are heated by the energy absorbed by the brake when braking. The temperature which is measured by the sensor is a function of the absorbed braking energy. By monitoring the temperature of the brakes, the braking force for the brakes integral to the vehicle can be monitored. The system described in said patent application measures the temperature and controls the brakes with a view to obtaining as even a temperature as possible for the brakes. This means that the system does not provide information on applied braking force at each of the brakes.
Should the vehicle comprise a number of axles bearing different loads and where the principle is adopted that each of the axles shall brake its own weight, brakes associated with different axles will absorb different amounts of braking energy. This means that the brakes will be heated to different degrees, in which case the system according to U.S. 2002/0138189 will cause a fault signal to be generated and the intended braking force to be redistributed.
Furthermore, the system according to U.S. 2002/0138189 will not be capable of enabling a redistribution between the applied braking force of a trailer coupled to the vehicle, in which the brakes are not monitored according to the system, and the vehicle's own brakes, since the system according to U.S. 2002/01 38189 ensures only that the brakes of the vehicle acquire substantially the same temperature. Should the brakes of the trailer absorb more than the weight of the trailer, the brakes of the tow car will get too cold, and should the brakes of the trailer absorb less than the weight of the trailer, the brakes of the tow car will get too hot. The correction of braking force distribution between trailer and tow car is commonly referred to as trailer adjustment.
The system described in 2002/0138189 is not configured to allow correct distribution of the braking force between the brakes of an unmonitored trailer and the brakes of a tow car.
In known systems, trailer adjustment is realized under the assumption that the friction coefficient of the brake device of the vehicle is constituted by a known constant. The friction coefficient varies between different braking devices, inter alia in dependence on how the brake has previously been used and in what environment the brake has been located in. This variation can be explained by the fact that so-called glazing can take place or by the presence of dirt and rust on the brake. Both glazing and dirt and rust deposits have a major effect on the friction coefficient of the brake.